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82815 GMCH
R
166
Datasheet
6.1.
XOR Tree Testability Algorithm Example
XOR tree testing allows users to check, for example, opens and shorts to VCC or GND. An example
algorithm to do this is shown in Table 24.
Table 24. XOR Test Pattern Example
Pin # from Figure 14
Vector
PIN1
PIN2
PIN3
PIN4
PIN5
PIN6
XOROut
1
0
0
0
0
0
0
1
2
1
0
0
0
0
0
0
3
1
1
0
0
0
0
1
4
1
1
1
0
0
0
0
5
1
1
1
1
0
0
1
6
1
1
1
1
1
0
0
7
1
1
1
1
1
1
1
In this example, Vector 1 applies all 0s to the chain inputs. The outputs being non-inverting, will
consistently produce a 1 at the XOR chain output on a good board. One short to Vcc (or open floating to
Vcc) will cause a 0 at the chain output, signaling a defect.
Likewise, applying Vector 7 (all 1s) to chain inputs (given that there are an even number of signals in the
chain), will consistently produce a 1 at the XOR chain output on a good board. One short to Vss (or open
floating to Vss) will cause a 0 at the chain output, signaling a defect. It is important to note that the
number of inputs pulled to 1 will affect the expected chain output value. If the number chain inputs
pulled to 1 is even, then expect 1 at XOR-out; otherwise, if odd, expect 0.
Continuing to Illustrate with the example pattern in Table 24, as the pins are driven to 1 across the chain
in sequence, XOR-out will toggle between 0 and 1. Any break in the toggling sequence (e.g., 1011) will
identify the location of the short or open.
6.1.1.
Test Pattern Consideration for XOR Chains 3 and 4, and 7 and 8
Bi-directional pins HLPSTRB (chain 3) and HLPSTRB# (chain 4), and AGP strobes AD_STB0,
AD_STB1, and SB_STB (chain 7) and AD_STB0#, AD_STB1#, and SB_STB# (chain 8) must always
be complementary to each other. For example, if a 1 is driven on to HLPSTRB, a 0 must be driven on
HLPSTRB# and vice versa. This will need to be considered in applying test patterns to these chains.